The concept of optical amplifiers, based upon the capability of certain materials, particularly on a macroscopic level, is well known. Thus, for example, it is known to place a pumping light source and a single crystal neodymium-yttrium aluminum garnet (ND:YAG) rod, several millimeters in diameter and several centimeters in length, in a tubular reflective cavity. For example, the light source and ND:YAG rod may be located, respectively, to extend along the two foci of a cavity having an elliptical cross section. In such an arrangement, light emitted by the light source and reflected from the cavity walls will impinge upon the ND:YAG rod. The light source is preferably selected to emit wavelengths corresponding to the absorption spectra of the ND:YAG crystal so that the neodymium ions of the crystal are inverted to an energy level above the upper lasing level. After inversion, an initial relaxation of the neodymium ions through phonon radiation yields an ion population at the upper lasing level. From this level, the ions will relax to a lower lasing level, emitting light of a wavelength which is characteristic of the ND:YAG material. Advantageously, the lower lasing level is above the ground level for the ions so that a rapid, phonon-emitting relaxation will occur between the lower lasing level and the ground level, enabling a high inversion ratio to exist between the upper and lower lasing levels within the pumped ions.
With the population so inverted, as is well known from laser technology, the ND:YAG will provide a very slow fluorescence, that is, random emission of incoherent light. This spontaneous radiation, however, has a minimal effect on the amplifying rod, since the average lifetime of ions in the inverted state is 230 microseconds, in ND:YAG.
If, after some of the neodymium ions of the ND:YAG rod have been inverted, a light signal at the lasing frequency is transmitted through the rod, the light signal will trigger the relaxation of the neodymium ions, causing coherent emission of stimulated radiation, which will effectively add to the transmitted signal, thus amplifying this signal.
The absorption length of the pumping illumination within the ND:YAG crystal (i.e., the length of material through which the illumination must traverse before about 65% of the illumination is absorbed) is typically in the range between 2 and 3 millimeters, and thus the ND:YAG crystals used in transverse pumping structures such as described previously have had diameters at least this large so that the crystal could absorb a substantial portion of the pumping radiation during the initial reflection from the cavity walls and passage through he crystal. If, during this initial traverse through the crystal, the pumping illumination is not absorbed, it is likely to be reflected by the cavity walls back to the light source, where it will be reabsorbed, generating heat in the light source and reducing the overall efficiency of the amplifier.
When such amplifiers are used in fiber optic systems, it has been thought necessary, because of the large difference in diameter between the optical fiber and the ND:YAG crystal, to use optical components, such as lenses, to focus light from the optical fiber into the ND:YAG rod, and the amplified light signal from the ND:YAG rod back into another fiber. Such optical systems require careful alignment and are susceptible to environmental changes, such as vibration, and thermal effects. Additionally, the optical components and the size of the ND:YAG rod make the amplifying system relatively large, and thus impractical for certain applications. Furthermore, the relatively large size of the ND:YAG rod introduces beam wander within the rod due to thermal effects. Thus, the signal from the input fiber optic element will traverse different paths through the rod, a characteristic which is temperature related and varies with time, so that the output light may be lost due to the fact that the fiber will accept only light within a small acceptance angle. Thus, as the beam within the ND:YAG rod wanders, the output signal may vary in an uncontrollable manner. Furthermore, the large size of the ND:YAG rod requires a large amount of input energy in order to maintain a high energy density within the rod. Such large pump power requires high output light sources, generating substantial heat which must be dissipated, typically by liquid cooling of the cavity.
While amplifiers of this type are useful in many applications, such as some communications applications, a use which puts severe limitations upon the amplification system is a recirculating fiber optic gyroscope. With such gyroscopes, an optical fiber, typically a kilometer or more in length, is wound into a loop, and a light signal is recirculated within the loop in both directions. Motion of the loop causes a phase difference between the counter-propagating light signals which may be used to measure gyroscope rotation. It is advantageous, because the phase shift induced in one rotation is relatively small and because periodic outputs relating to rotation are required, to recirculate input light within the loop as many times as possible.
In traversing a kilometer of optical fiber, an optical signal will typically lose 30 to 50 percent of its intensity. An amplifier, if capable of amplifying the bidirectional counter-propagating light signals, would permit a light signal to propagate many more times within the loop, if the amplifier were placed in a series with the loop, and provided a gain equal to the propagation loss.
Unfortunately, the relatively large size, high power requirements caused by relatively inefficient performance, beam wander effects, and cooling requirements of prior art ND:YAG rod amplifiers makes such amplifiers relatively impractical for high accuracy gyroscopes. These factors, of course, also limit the utility of such amplifiers in other applications, such as communication networks.